Currently, it is common to use sets of piston (plunger) and cylinder driven by electric motors for application, for instance, to gas compressors of refrigeration equipments, such as industrial/commercial/domestic refrigerators, freezers and air-conditioning devices.
In these types of compressors, the electric motor drives the piston which, in turn, moves inside the cylinder in an axial swing (back and forth) movement, so as to compress and decompress the gas. Normally, in the header of this cylinder, valves for suction and discharge of gas are positioned, which valves respectively regulate the input of gas at low pressure and the output of gas at high pressure from inside the cylinder. The axial movement of the piston inside the cylinder of the compressor compresses the gas admitted by the suction valve, raising its pressure and discharging it through the discharge valve to a high pressure zone. Alternatively, there are configurations of compressors in which the suction valve is positioned on the piston itself.
FIG. 1 illustrates a graph that relates the input pressure (suction) of a gas compressor to its output pressure (discharge), wherein curve ER represents a standard curve of the refrigeration equipment and curve C represents a standard curve of the compressor operating isolated from any refrigeration equipment or system. It is worth noting that curve ER represents the behavior of the refrigeration equipment in the pull down period of the compressor (time so that the internal temperature of the refrigeration equipment decreases until it reaches a pre-established temperature or time passed from the start-up of the compressor until it reaches the situation of regime).
Line P represents the pressure of equalization of the system in view of the gas cargo and the room temperature. It is worth noting that in line P, the suction pressure (input) and discharge pressure (output) are the same. Thus, if the relation between the suction pressure and the discharge pressure is not compatible with line P upon the start-up of the compressor, it will be under condition of blocked rotor, that is, the compressor will not be able to start-up even being energized and, consequently, the refrigeration equipment will not work as expected.
In curve ER, it can be observed that the discharge pressure quickly increases until it reaches approximately 11 bar, whereas the suction pressure decreases at lower rates until approximately 3.5 bar. From this point (first inflexion point in the curve), the discharge pressure increases at lower rates up to a maximum value (second inflexion point), around 14 bar, to, then, (third inflexion point), slowly decrease until a value of permanent regime. In this period, the suction pressure starts quickly decreasing until a value of approximately 1.3 bar, slowly increasing again with the discharge pressure up to a peak of approximately 1.9 bar, from which it starts softly decreasing until a condition of balance is achieved (regime).
In the condition in which curve ER intercepts curve C, there is the undesired situation when the compressor overturns, once the electric motor does not have enough torque to provide the proper operation of the compressor. This interception (intersection) may occur between the moment the compressor starts-up and the first inflexion point of curve ER. After overturning, the motor stops working and the relation between the suction pressure and discharge pressure does not obey line P and, therefore, the motor rotor will be blocked, and the compressor will not be able to start-up. Starting-up the compressor will only be possible when the suction pressure and the discharge pressure are equalized, that is, when the relation between these pressures is in accordance with line P.
Therefore, a problem commonly noted in the electric motors of compressors is when they overturn upon pull-down. Moreover, under this condition of blocked rotor, the thermal protector of the electric motor will be requested, which is evidently an undesired situation.
Additionally, when the supply of electric power to the compressor is shortly interrupted, the suction pressure will not be equalized with the discharge pressure (condition established by line P) and, consequently, the compressor will not be able to start-up. Because of that, the thermal protector of the electric motor will be requested, and it will be necessary to wait a certain time so that the suction and discharge pressures are equalized.
Thus, in view of all the problems abovementioned, the electric motors for compressors are currently over-dimensioned, so as to place curve C far from curve ER so that their operation is not impaired and, therefore, it is necessary to use a motor with higher capacity, more expensive, which also occupies a larger space (over-dimensioned motor to avoid the intersection between curves C and ER).
Furthermore, considering that the compressors are normally positioned over springs, it is common to observe their excessive vibration and a high level of noise resulting mainly from the impact of the motor on the casing, upon its turning-off (stop), once, as the movement of the piston does not immediately stop when it is required to stop due to the inertial force, it keeps trying to compress the gas. However, after a certain time, this inertial force is no longer enough to provide the opening/closing of the valves. This way, the gas is retained inside the cylinder and, therefore, its compression is not appropriately and softly performed, causing the compressor to undesirably vibrate. Because of that, many compressors have casings with over-dimensioned external dimensions, to place them as far as possible of the motors, in order to avoid the impact thereon. However, this over-dimensioning of the casing makes it difficult to transport the compressor, apart from requiring a larger space for its installation inside the refrigeration equipment. Furthermore, the space created between the casing and the motor makes it easier to break internal pieces, parts, and components of the compressor, when it is transported.